docs: def read_instance(self, cls, sdmxobj, offset=None, first_only=True):
          '''
          If cls in _paths and matches,
          return an instance of cls with the first XML element,
          or, if first_only is False, a list of cls instances 
          for all elements found,
          If no matches were found, return None.  
          '''
          if offset:
              try:
                  base = self._paths[offset](sdmxobj._elem)[0]
              except IndexError:
                  return None
          else:
              base = sdmxobj._elem
          result = self._paths[cls](base)
          if result:
              if first_only:
                  return cls(self, result[0])
              else:
                  return [cls(self, i) for i in result]